Cylinders of a multiple cylinder engine are arranged to sequentially have their power strokes, which in the art is known as the firing order. The firing order of an engine is primarily determined by the positioning of the cylinder and cranks on the crank shaft, where rotation of the crankshaft causes the reciprocal operation of pistons within the cylinders. For example, a four-cylinder four-stroke engine has power strokes that occur at one-hundred and eighty degree intervals of crankshaft rotation such that the pistons move in pairs within the cylinders. For instance, the pistons in the first and fourth cylinders move as a pair whereas the pistons in the second and third cylinders also move as a pair. As such, in a four cylinder engine having a firing order of 1-3-4-2, when the first piston begins its power stroke the piston in the fourth cylinder begins its induction stroke. Alternatively, the second piston begins its exhaust stroke while the third piston begins its compression stroke.
When a cylinder enters the exhaust stroke the exhaust gases are expelled from the cylinder into an exhaust manifold via exhaust valves. The exhaust manifold collects exhaust gases from each cylinder where it directs the gases to an exhaust pipe or turbocharger within the vehicle. In a typical configuration, an exhaust manifold comprises a number of branches that connect with and extend from individual cylinders at the cylinder head. Therefore, the exhaust manifold comprises multiple inlets and outlets that connect to an exhaust pipe which thereby facilitates eviction of exhaust gases, after treatment, to the atmosphere. The exhaust manifold may be cast or fabricated either as a separate part to the cylinder head of an engine or as an integral part of the cylinder head casting. In the case of an integrated exhaust manifold, the length of the exhaust manifold port runners or branches are kept short to enable the head to be cast and in order to save costs on material.
One problem associated with the exhaust manifold layout is that the exhaust gases from one cylinder can be discharged into another cylinder if any exhaust valve overlap exists. As such, an integrated exhaust manifold may suffer from interference between cylinders, particularly where the exhaust branches from adjacent cylinders (for example the exhaust branches from the third and fourth cylinders) in the manifold are relatively short and the cam duration period is particularly long (e.g., greater than one hundred and eighty degrees crank rotation for a four cylinder engine). Therefore, if the cam duration is longer than 180 degrees of crank rotation, two cylinders most likely have exhaust ports open at the same time such that one port is closing while another port is opening. This results in exhaust gas flowing from the higher pressure cylinder (e.g., the cylinder that has just begun its exhaust stroke) into an adjacent cylinder where a lower cylinder pressure exists as the exhaust port is closing. The consequence of exhaust gas flowing between cylinders is that the fuel mixture within the cylinder may be contaminated by the presence of exhaust gases that further affect combustion efficiency at the next induction stage of the engine cycle.
Attempts to overcome the discharge of exhaust gases from one cylinder to another have been made and include blocking of the discharge in the manifold assembly. However, such solutions involve a redesign of the exhaust manifold layout. Furthermore, in the case of an integrated exhaust manifold, the scope for modifying the exhaust branches are limited due to a desire to maintain the compact arrangement provided by an integrated manifold. It is therefore desirable to reduce the possibility of exhaust gases being discharged from one cylinder to another while also preventing the discharge from one cylinder from blocking the discharge from another cylinder. It is also desirable to reduce the possibility of abnormal combustion, or engine knock, due to the mixture of fuel and exhaust gases in the cylinder following the intake of fuel.
The inventors have recognized disadvantages with the approaches noted above and herein describe a camshaft for a multiple cylinder four-stroke internal combustion engine, wherein each exhaust lobe is arranged to operate an exhaust valve of an associated cylinder upon rotation of the camshaft so that peak lift of a first exhaust lobe is angularly displaced relative to peak lift of a second exhaust lobe by an angle of cam rotation greater than an angle defined by a full revolution of the camshaft divided by the number of cylinders of the multiple cylinder engine. By increasing the angle between peak lift of exhaust lobes associated with physically adjacent and successive firing cylinders, the exhaust port of an earlier firing cylinder begins to close earlier than a conventional symmetric arrangement and opening of the exhaust port of the subsequently firing cylinder may be delayed. As such the crossover or overlap period when two exhaust ports are open may be reduced. Therefore, the period of camshaft rotation during which transfer of exhaust gas from one cylinder to another is also reduced.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.